In the art there has been suggested various methods for thermo treatment of wood as will be explained below. The purpose of subjecting wood to a thermo treatment is that it has for a long time been known that by treating wood under a certain temperature regime increasing the temperature for a period of time and thereafter reducing the temperature back to ambient temperature the wood attains some improved qualities. For example the durability as well as the insulating properties of the timber are improved. Laboratory tests have shown that this is due to a structural reordering of the molecular structure of the wood such that the wood from having a more or less random molecular fibre structure due the thermo treatment is reorganized to have a much more structured and linear fibre structure at the molecular level which provides for the improved characteristics.
These aspects are clearly disclosed and discussed in the “Thermo Wood® Handbook” published by the Finnish Thermo Wood Association in 2003. This book is widely considered as the reference work when it comes to thermo treatment of wood. According to the disclosure the process is divided into three phases where the wood which is placed in a treatment chamber is subjected to an increase inside the treatment chamber in two steps, first up to a temperature of approx. 100° C. for a first period and thereafter to a temperature of approx. 130° C. for a second period.
The purpose of the first phase is to dry out the wood and this phase lasts approx. 36 hours. In the second phase the temperature is further increased to between 185° C.-250° C.
The elevated temperature is maintained for approx. 16-17 hours in order for the wood to be subjected to the modification process as described above.
Finally, in the third phase a cooling and moisture conditioning phase is carried out where once the temperature has fallen below 80-90° C., a remoisturing of the wood takes place such that the moisture content in the treated and finished wood is in the range of 4-10% by weight. The third phase depending on the type of wood being treated typically takes 18-28 hours.
A method as discussed above is for example disclosed in EP 2 998 087 with a few modifications. According to the method in EP 2 998 087 wood is introduced into a treatment chamber in which the temperature is increased up to 173° C. and maintained for 3-5 hours. Thereafter the temperature is decreased to approx. 20° C., and the wood is transferred to an autoclave. In the autoclave linseed and mineral oil is introduced and allowed to penetrate the wood, which thereby becomes impregnated.
Wood mainly consists of three different components, namely hemicelluloses, celluloses and lignin. These materials have different characteristics and as such they react differently during the heat treatment. Hemicelluloses is special in that in the first part of the heating of the wood sample the modification of hemicelluloses is endothermic meaning that heat is transferred and absorbed by the wood until a certain temperature is reached.
This certain temperature is depending on the type of wood and thereby also the contents of hemicelluloses which varies depending on the species and the growth conditions for that particular species as well as the moisture content and the pressure, but it will typically be around 230° C.
At this temperature the modification of hemicelluloses turns from an endothermic process to an exothermic process, i.e. more energy is generated than what is added to the hemicelluloses component of the wood. At the same time the celluloses will have been modified and will still he undergoing modification. Typically, the cellulose part of a wood sample will be substantially larger than the hemicelluloses part, and a such a substantial part of the wood has been modified at this stage.
A number of drawbacks, however, are associated with the prior art methods and procedures.
Firstly, the procedure takes a very long time thereby reducing the output from a process plant. Typically, a treatment of a batch of wood with prior art methods takes from 24 hours and up to 36 hours depending on the wood and how aggressive the modification process is pursued.
The very long process time and thereby the low turnover in the machinery naturally increases the cost of the modified wood due to the long process time. Furthermore, traditional modification processes use steam and heated steam in order to increase the heat inside the wood and there thereby activate the modification process. As there is already moisture inside the wood and the wood is not absolutely homogenous there will be a non-even distribution of moisture inside the wood and at the same time the wood may not have a completely homogenous structure.
This does cause problems to the quality of treated wood in that as the moisture inside the wood is heated, steam will be generated and due to the variations both in moisture content and the wood structure as well as the variation of density in the wood to be treated the internal pressure inside the wood due to the heating will cause cracks and other detrimental side effects during the treatment. As the treatment chamber has a relative high steam pressure, the built up pressure inside the wood cannot dissipate slowly, but will eventually cause a small steam explosion, potentially causing cracking or other damage. At the same time miscolouring of the surface may be a result.
In order to improve this, it has been suggested in JP2013180460 to replace the air and steam inside the treatment chamber by a super critical carbon dioxide atmosphere. Super critical carbon dioxide is in the Japanese reference defined as carbon dioxide beyond a critical point which is described as being 31° C. at 7.4 MPa.
When the carbon dioxide is in a super critical state, it acts like a fluid and as such together with the very high pressure (above 74 bar) it replaces the moisture inside the wood structure. In order to remove the moisture from the wood it is necessary to further heat the super critical carbon dioxide atmosphere in order to transform moisture, typically water, from its liquid to its gaseous state, i.e. steam. This in turn causes the pressure to increase even more. This process therefore has a number of drawbacks, firstly the vessel in which the process is to be carried out must be extremely strong in order to be able to withstand the very elevated pressure inside the treatment chamber.
Furthermore, any generation of steam exposed to such a high pressure will have a severely detrimental effect on any imperfections such as cracks, nuts and the like in the wood, thereby causing the wood to crack or split.